An intervertebral disc is a pad of cartilage-type material situated between spinal bones. Each disc serves as a connector, spacer, and shock absorber for the spine. A soft, jelly-like center is contained by outer layers of fibrous tissue. Healthy discs facilitate normal turning and bending. Trauma or injury to the spine can cause discs to tear, bulge, herniate, and even rupture. This can be quite painful, as the soft center of the disc leaks, putting pressure on the adjacent nerve roots and spinal cord.
A damaged disc can cause nerve dysfunction and debilitating pain in the back, legs and arms. Typical treatments that provide relief and allow patients to function again include back braces, medical treatment, physical therapy and/or surgery to remove the disc.
A conventional surgical solution removes the injured or degenerated disc and promotes new bone growth in the space to fuse the adjacent vertebrae. Such surgery can be highly invasive and may require two relatively large incisions. A first incision may be made in the front of the body so that the damaged disc can be removed. The second incision may then be made in the back so that, for example, connecting rods and anchor screws can be attached to the vertebrae to stabilize them long enough for the new bone to grow. This type of surgery typically results in recovery periods that can extend as long as six months.
For the purpose of achieving long term stability in a segment of injured spine, a fusion (the joining together of two or more bones via a continuous bridge of incorporated bone) may be performed. Interbody fusion, wherein the disc is partially excised and bone placed within the space previously occupied by the excised disc material (between adjacent vertebrae), is one typical type of fusion. Interbody fusion is performed for the purpose of restoring a more “normal” spatial relationship, and to provide for stability; short term by mechanical support, and long term by the permanent growth of bone from vertebra to vertebra.
For fusion to occur within the disc space, in certain procedures, it is necessary to prepare the vertebrae to be fused by penetrating, or cutting into, the hardened outside cortical plates of bone (the endplates) to allow an interposed bone graft to come into direct contact with the more vascular cancellous (spongy) bone, and to thereby stimulate the body to heal this induced, but controlled, “injury” by both bone production and “creeping substitutions” of the graft to create a continuous segment of bone between the opposed vertebral surfaces.
Following the removal of a damaged disc, if an implant, such as a bone graft, is not placed in the intervertebral space, collapse may occur, which may result in damage to the nerves; or the space may fill with scar tissue and eventually lead to a reherniation. However, the use of bone to fill the space is sometimes suboptimal because bone obtained from the patient requires additional surgery and is of limited availability, and if obtained from another source, lacks living bone cells, carries a risk of infection, and also is limited in supply. Furthermore, regardless of the source of the bone, it may have marginal biomechanical characteristics and may lack means to either stabilize itself against dislodgement or to stabilize the adjacent vertebrae.
There have been extensive attempts to develop an acceptable disc prosthesis (an artificial disc). Such devices would be used to replace a damaged disc, to restore the height of the interspace, and to restore the normal motion of that spinal joint. Examples include a flexible disc implant, a flexible disc replacement with file-like surface projections to discourage dislocation, and a bladder-like disc replacement with two opposed stud-like projections. Although such devices are placed within the intervertebral space following the removal of a damaged disc, they may result in eventual fusion or fixation of the spine.
Related to disc prosthetics are those devices used to replace essentially wholly removed vertebrae (e.g., corpectomy devices). Such removal is generally necessitated by extensive vertebral fractures, or tumors, and is not associated with the treatment of disc disease. Due to the removal of the entire vertebra, intervertebral disc-replacements are not feasible. Therefore, these implants perform as temporary structural members mechanically replacing the removed vertebrae (not removed disc), and do not intrinsically participate in supplying osteogenic material to achieve cross vertebrae bony fusion. Typically, use of these devices will be accompanied by further surgery consisting of a bone fusion procedure using the conventional techniques.
Similarly, other devices are designed to be placed within the vertebral interspace following the removal of a damaged disc, and seeking to eliminate further motion at that location. One such device is contained in U.S. Pat. No. 4,501,269 issued to Bagby, which describes an implantable device and instrumentation. The method employed is as follows: a hole is bored transversely across the joint, a hollow metal basket of larger diameter than the hole is impacted into the hole, and the hollow metal basket is filled with the bone debris generated by the drilling.
Implants such as those disclosed in the Bagby patent were impacted against resistance to achieve vertebral distraction, and were, therefore, susceptible to forceful dislodgement by the tendency of the two distracted vertebrae to return to their original positions, squeezing out the device. Conversely, the next generation of devices approved by the FDA and sold commercially were threaded cylinders, usually referred to as “cages”. These cages typically are manufactured from biocompatible metals, such as titanium. These implants, are typically screwed into place. Because no unscrewing force exists between the vertebrae, compression alone cannot dislodge the implant. The implant is, therefore, more stable.
Spinal implants having threaded cages facilitate a less traumatic insertion into the intervertebral space. Such devices can be securely screwed into place, and often possess highly specialized locking threads to make accidental dislodgement impossible. Because of the proximity of the spinal cord, spinal nerves, and blood vessels, any implant dislodgement might have catastrophic consequences.
An implantation procedure involves numerous steps. According to one method of implantation, these steps may include the presurgical measurement of the vertebrae, selection of an appropriate implant size, and determination of a desired distraction of the vertebrae in order to achieve a desired alignment of the spine. A discectomy or partial discectomy also may be performed, removing part or all of the disc. Alternatively, no disc material need be removed. Incisions and retractions are performed to expose the injured area of the spinal column. A distractor is then inserted and impacted into the intervertebral space. After distraction, a surgical tube is positioned over the implant area. Once a hole is drilled, various other steps are performed in order to tap (i.e., produce threads in) the adjacent vertebrae, for threading of the implant into the hole. It should be noted that these steps are merely a representative example of what may occur during an implantation surgery and individual steps may be altered or omitted, the order of the steps may vary and additional steps may be included in the process.
One exemplary implantation device that is currently in use is sold under the trade name BAK™ INTERBODY FUSION SYSTEM by Zimmer Spine Inc. (Minneapolis, Minn.). In this technique and apparatus, a hollow cage, about an inch long, is implanted through a small incision into the disc space between two vertebrae. The surgical invasion is highly reduced from the previous methods and patients recover much faster. In this method, the disc need not be removed entirely, but rather may be drilled out in two bilateral bores. An implant is placed in each bore space between the adjacent vertebrae to stabilize the spine. Morselized bone is harvested from the patient and packed inside the implant. Over time, new bone will fill the inside and outside of the implants and fuse the vertebrae. The degenerated disc need not be removed completely, because the bored out disc does not block the formation of new bone between two opposite sites on the adjacent vertebrae.
FIG. 1 shows a prior art surgical implant driver 300 having a handle 366 and a shaft 350. An implant 390 is attached to the distal end of the shaft 350 by means of a threaded portion 353. A protruding central area 362 on the distal end of the shaft 350 mates with a groove 364 of the implant 390 to ensure that the implant is not unscrewed from the threaded portion 353 when the driver is rotated.
As shown in FIG. 2, once a disc 400 and adjacent vertebrae 410 have been prepared for the implant 390, the shaft 350 of the driver is placed in a guide tube 140 that previously has been positioned in the appropriate location in the surgical area. After the implant 390 has traversed the surgical tube 140 and contacted the opening of the bore, the shaft 350 is rotated to align the threads 392 of the implant 390 with the threads cut into the vertebrae 410 as part of the implantation process. Further rotation of the shaft 350 screws the implant 390 into the bore until a desired depth has been reached. As shown in FIG. 3, the shaft 350 is detached from the implant 390 and the guide tube 140 is removed from its position in the surgical area, leaving the implant 390 in a predetermined intervertebral position. Because such implants often are installed in pairs, a similar procedure typically will be performed on the other side of the spine.
Although FIGS. 2-3 demonstrate a posterior implantation in the lumbar spine, spinal implants may be installed from other approaches. Furthermore, the implantation procedure may also be performed anteriorly or as a laparoscopic procedure.
Metal cages, although strong, are largely radiopaque. Because of this, the ability to detect fusion through diagnostic methodologies, such as x-rays, is greatly reduced with such devices. Radiolucent cages, made from polymers such as polyether ether ketone (i.e. PEEK), solve this problem. However, because such cages may not be as strong as those made of titatnium, there is an increased risk of fracture during implantation or removal, in the event that excessive torque loads are exerted on the implant. Accordingly, a need exists for a surgical driver that is better able to manage and direct these torque loads, in order to minimize the risk of implant fracture.
Because, in certain circumstances, significant force (e.g., torque) may be exerted on the implant during surgery, it is possible for implants to become deformed or break. In addition, such deformation or breakage will make the implantation or removal of the damaged implant difficult. As noted above, these challenges are exacerbated when radiolucent implants are used, because of their potentially reduced load bearing capacities as compared to titanium inserts. Common reasons for breakage may include misalignment of the implant, insufficient preparation of the implant hole, and/or improper connection to the surgical driver. For example, misalignments of the implant, relative to the implant bore and/or the driver may result in variable-axis forces being applied to the implant. Moreover, these conditions sometimes are difficult to detect and/or fully appreciate during surgery. Thus, because human operators will at times apply such variable-axis forces to the implant through a rigid surgical driver, such occurrences heretofore have not been completely eliminated. Accordingly, there exists a need to provide a surgical implant driver that will minimize the transmission of such variable-axis forces to the implant.